Changes in the fiber orientation distribution of a nonwoven web cause the physical properties of the final product to vary. For example, paper may suffer from problems such as stack lean and corner to corner curl unless the mean fiber orientation distribution is uniformly in the machine direction. Also, unless the fiber orientation distribution is sufficiently random, there may be wrinkling or dimensional instabilities in the nonwoven web.
The fiber orientation distribution in paper is currently measured off-line by the zero span tensile method in a laboratory. R. J. Votava, TAPPI 65(4), 65 (1982). Small strips (usually 15 mm in width) are cut from a sample of paper in the machine direction, in the cross-direction and, depending on the accuracy required, at various angles in between. The tensile strength of each strip is then measured. Another off-line technique involves a visual count in a microscope of stained fibers. O. J. Kallmes, TAPPI 52(3), 482 (1969). Both tests are time-consuming, necessitate sample damage and require a certain degree of skill in preparing the sample.
There is another off-line method for the determination of fiber orientation by x-ray diffraction that is quicker than the zero span tensile or the microscopic measurement methods. R. E. Prud'Homme, et al., Applied Polymer Science 19, 2609 (1975). That method requires a prior knowledge of fibril angle distribution for accurate measurements; in addition, there is still some sample damage and some sampling skill is still needed. None of the foregoing methods are adaptable to on-line use.
A method for determining fiber orientation using laser light was introduced in 1970. L. Rudstroem, et al., Svensk Papperstidn 83(5), 117 (1970). When a focused coherent beam of light is transmitted through a nonwoven web, such as a sheet of paper, a diffraction pattern results which corresponds with fiber orientation. Rudstroem, et al. detected the pattern with a photomultiplier tube (PMT) behind a narrow rotating aperture. Hartig improved on that method by detecting the diffusion pattern with an arrangement of six linear photo-diode arrays. W. Hartig, West German Patent DE No. 3414558, issued Oct. 24, 1985.
The laser detection methods require that the detector always be in alignment with the laser beam. When measuring wide sheets on line, the laser and detector must be independently mounted and then precisely aligned. An independent mounting may allow the laser beam to move with respect to the detector and the pattern may move out of range of the detector. This problem is further compounded by the inherent vibration involved in the manufacture of the non-woven sheet. Consequently, these methods have not been adapted to general on-line use.
Therefore, it is an object of the present invention to provide an on-line method and apparatus for detecting and analyzing the diffusion pattern of laser light transmitted through a moving sheet of nonwoven material, such as paper.
It is a further object to use that diffusion pattern to provide an on-line determination of the fiber orientation distribution in the moving sheet of nonwoven material.
It is another object of the present invention to provide an improved method for manufacturing paper.